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Pain Management
for the Pregnant,
Lactating, and
Neonatal to Pediatric
Cat and Dog
Karol A. Mathews, DVM, DVSc
The science and management of pain is extremely broad, and investigation into all
aspects takes time. Because most cats and dogs requiring analgesia to manage
pain are mature and not pregnant or lactating, it stands to reason that this majority
would be studied and emphasized in most textbooks and journal articles. Unfortu-
nately, neonatal, pediatric, and pregnant or lactating animals have received little atten-
tion in veterinary investigations. As a consequence, analgesics are often avoided in
these animals, because the effects of these drugs on the developing fetus, the nursing
animals, and developing young are not known to many. The purpose of this review is to
focus on commonly used analgesics and their safety in these animals and to touch on
the associated pharmacologic aspects of these drugs.
ANALGESIA FOR PREGNANT DOGS AND CATS
Pregnant animals may incur injury, undergo a surgical procedure, or experience
chronic pain, requiring management with analgesics. Unfortunately, there are no clin-
ical studies investigating the safety of any analgesics in the pregnant dog or cat.
Although many peri-cesarean section analgesic and anesthetic regimens have been
recommended and used with no apparent ill effects, the more subacute to chronic
use of analgesics in pregnant dogs and cats has not been studied. A comprehensive
source of information cited in relation to analgesia in pregnant women classifies all
drugs to risk factor category A, B, C, D, or X based on evidence of risk to the fetus
and nursing infant.
1
The A category declares no risk to the fetus, whereas B, C, and
D represent progressive risk to the fetus and recommendations for extenuating use.
Category X indicates contraindication in women who are or may become pregnant.
Emergency and Critical Care Medicine, Department of Clinical Studies, Ontario Veterinary Col-
lege, University of Guelph, Guelph, Ontario, Canada N1G 2W1
E-mail address: kmathews@ovc.uoguelph.ca
KEYWORDS
Pregnant Lactating Neonates Pediatric Pain Analgesia
Vet Clin Small Anim 38 (2008) 1291–1308
doi:10.1016/j.cvsm.2008.07.001 vetsmall.theclinics.com
0195-5616/08/$ – see front matter ª2008 Elsevier Inc. All rights reserved.
Author's personal copy
Because of a lack of appropriate studies in animals and human beings, no drugs are
listed in category A. Many studies in pregnant women have shown minimal to no fetal
or neonatal compromise, however, with some opioids administered for variable
periods.
The pharmacologic features of pregnant animals differ from those of the nonpreg-
nant animal; various physiologic changes associated with the maternal-placental-fetal
unit can alter pharmacodynamics, pharmacokinetics, and distribution to the fetus.
2
The maternal factors that may alter drug absorption in women are decreased gastro-
intestinal motility and esophageal reflux and vomiting in addition to increased cutane-
ous blood flow, which may enhance absorption of transdermally administered drugs.
2
Whether these factors occur with some frequency, or are applicable to dogs and cats,
is not known. Cutaneous blood flow potentially may increase in the late stage of cat
and dog pregnancy, and enhanced absorption of transdermal medication may occur
in this setting. As total body water is increased with distribution throughout the mater-
nal tissues, amniotic fluid, placenta, and fetus, the volume for distribution of drugs is
also increased.
3
Total body fat may also be increased, resulting in a larger volume of
distribution for lipid-soluble drugs with less available in the plasma. Reduced serum
albumin, which may occur in pregnancy in women, could result in more free normally
protein-bound drug, which would then be available for action on maternal receptor
sites and transport across the placenta to the fetus. In dogs and cats, however, albu-
min levels may drop to low normal values, but it is thought that this is secondary to the
increased plasma volume and dilution in normal pregnancy and is not attributable to
reduced albumin load (Catherine Gartley, personal communication, 2005). Hepatic en-
zymatic activity may be altered, and renal function is gradually increased with
increased elimination of water-soluble drugs and metabolites.
2
The placental barrier is considered to be a lipoprotein; therefore, drugs with high
lipid solubility are permeable.
4
Lipophilic compounds diffuse passively along a con-
centration gradient to the fetus. Equilibrium is reached as the concentration increases
within the fetus, limiting further transport. Drugs that are polar, ionized, protein bound,
or water soluble are less likely to cross the placenta into the fetus. Should free drug
bind immediately to fetal proteins, a favorable concentration gradient persists until
protein binding is saturated and increased free drug equilibrates with the maternal
plasma concentration. Molecules smaller than 600 d, which applies to many pharma-
cologic compounds, readily cross the placenta. The placenta is an enzymatically
active organ. Cytochrome P-450 enzymes, N-acetyltransferase, glutathione transfer-
ase, and sulfating enzymes can alter the activity of drugs into their active or inactive
forms with variable activity in the fetus. Blood flow also determines the rate of drug
entering the placenta, and increased or decreased placental flow influences delivery
of drugs to the fetus.
4
As the placenta ages, its thickness decreases, facilitating further
diffusion into the fetus. In vitro studies examining transfer of morphine across the term
human placenta have shown that the cotyledon acts as a storage depot for morphine
and that morphine is released for approximately 60 minutes after the maternal admin-
istration of the drug ceases, effectively prolonging fetal exposure to morphine.
5
Because of species differences in anatomy, the site of potential placental storage of
analgesics will differ in dogs and cats, because these species do not have placental
cotyledons.
The stage of gestation of the fetus influences the effects of the various analgesics.
The same drug administered in the early stage has a different effect when ad-
ministered at the later stages of gestation.
1,2
The human fetal liver can perform
many enzymatic and metabolic activities as it matures; however, it cannot perform
glucuronidation, which is important for the metabolism of many lipophilic drugs,
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such as some opioids.
1
These effects would be similar to those in our veterinary
patients. Unlike the human fetus, the liver of the dog fetus (not known for cats) has
no drug-metabolizing capabilities; therefore, elimination of drugs from the canine fetal
circulation is by means of fetal immature renal mechanisms or diffusion back through
the placenta to the mother.
6
Fetal body water and albumin increase, and body fat
decreases, during development, all of which influence plasma concentrations
of various drugs.
2
Opioids
Currently, opioids are the analgesic of choice in pregnant women and animals. With
prolonged use (several weeks) during pregnancy, however, the fetus may be adversely
affected. An increased incidence of babies born with low birth weight and behavioral
deficits has been reported in human mothers taking opioids during pregnancy. Similar
findings have been reported in laboratory animals.
7
The behavioral problems incurred
with chronic use may be a result of reduced nervous system plasticity secondary to
the opioid action on the development of normal synaptic connections, neurotransmit-
ter production, and metabolism.
8
Chronic use during pregnancy would be extremely
rare in veterinary patients. Based on the human literature, short-term opioid analgesic
administration should not, and does not, seem to be a problem in veterinary patients.
Again, long-term use may result in adverse effects on the fetus; however, the benefit to
the mother must be considered. Of interest, a review of the addiction medicine litera-
ture, with experience gained from pregnant women seeking recovery from opioid
addiction, concluded that methadone seemed to be safe for the treatment of pain dur-
ing pregnancy.
2
Methadone is a good analgesic with efficacy comparable to that of
morphine. Only parenteral methadone can be administered to dogs, because this opi-
oid is not absorbed by way of the oral route in this species. Oral absorption has not
been reported in the cat.
Studies investigating the transplacental transfer and metabolism of buprenorphine
in the isolated placenta observed low transplacental transfer of buprenorphine to the
fetal circulation with a single ‘‘dose’’.
9
Because buprenorphine is deposited into the
intervillous space, it acts as a depot and transplacental transfer to the fetal circuit is
low. The direct effects of buprenorphine on the fetus depend on its concentration in
the fetal circulation. Because less than 10% of placental buprenorphine, which is
slowly released from the placenta, reaches the fetal circulation, little would be avail-
able to the fetal circulation.
9
With repeated administration, however, an increased de-
pot of the drug would result in continuous release into the fetal circulation, which may
contribute to neonatal withdrawal in a small number of neonates.
10
Another study examining the rate of transfer from the maternal circulation of fentanyl
reported that fentanyl rapidly crossed the placenta and entered the fetal brain during
the first and early second trimesters in aborted human fetuses.
11
The concentration of
fentanyl was still present in the fetal brain after clearance from the maternal blood. A
study investigating human fetal and maternal plasma opioid concentrations after epi-
dural sufentanil-bupivicaine or fentanyl-bupivicaine mixtures administered for analge-
sia during labor and delivery noted that sufentanil placental transfer was greater than
that of fentanyl.
12
There was significant reuptake of sufentanil to the maternal circula-
tion, however, which may considerably reduce neonatal opioid exposure. In this study,
fentanyl administration was associated with lower neurobehavioral test scores at
24 hours of life, although none of the neonates had clinically significant depression.
The investigators concluded that both drugs are acceptable for use with epidural
bupivicaine during labor but that reduced neonatal opioid exposure with sufentanil
suggests that it may have some advantages over fentanyl.
12
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No reports linking therapeutic use of morphine with major congenital defects have
been published.
1
Maternal addiction to morphine with subsequent neonatal with-
drawal syndrome is well documented, however. Morphine was widely used during la-
bor in women until the 1940s, when it was replaced by meperidine. The clinical
impression was that less respiratory depression was noted with meperidine when
compared with that when morphine was used.
1
For continuing analgesia, however,
morphine is recommended and not meperidine.
Opioids are frequently used in veterinary medicine to control pain associated with
cesarean section. For the most part, puppies and kittens are successfully delivered
and vigorous. If the puppies or kittens are depressed after delivery, a small drop of nal-
oxone placed sublingually should reverse the depressant effects of the opiate. Repeat
dosing in 30 minutes may be required if the neonates become depressed again. If con-
tinual renarcotization in the newborn is a concern, the owner should be given instruc-
tions on sublingual administration of a drop of naloxone dispensed in a tuberculin
syringe. Other potential causes for perioperative depression must be considered if
the cesarean section was not routine.
Opioid Antagonist
When ‘‘overdose’’ or adverse effects of an opioid are noted in the mother after any sur-
gical procedure required during pregnancy, reversal by titration with naloxone is effec-
tive. One approach is to combine naloxone (0.4 mg/mL) 0.1 mL or 0.25 mL (for larger
animals) with 10 mL saline and titrate at 1 mL/min only until unwanted affects are elim-
inated; with this technique, the analgesia still persists. Because naloxone may only last
for 30 minutes, however, redosing in the same manner may be required.
It is not known whether the placenta retains naloxone. If this is the case, naloxone
may have the effect of counteracting prolonged fetal exposure to morphine, because
placental leaching of the drug would negate the need for repeated maternal naloxone
dosing strictly as a fetal protectant.
5
It is known that naloxone does not alter the trans-
fer or clearance of morphine across the placenta and that naloxone’s effects are likely
antagonism of morphine by direct actions on fetal m-receptors.
5
When opioid analgesia is required, as in any other painful situation, one should dose
to effect (Table 1) and treat the underlying problem. It is also important to ensure that
there are no other stresses and the patient has an environment that is comfortable,
clean, and at normal ambient temperature.
Nonsteroidal Anti-Inflammatory Analgesics
The nonsteroidal anti-inflammatory analgesics (NSAIAs) are used extensively in hu-
man and veterinary medicine. In addition to administration for pain management,
NSAIAs are prescribed to reduce significant abnormal right-to-left shunting of blood
across the ductus arteriosus or foramen ovale when this is identified before birth of
human babies.
13
The NSAIAs inhibit cyclooxygenase (COX) production, with a subse-
quent reduction in prostaglandin synthesis, which normally maintains ductal patency
and regulates the pulmonary vasculature.
14
Adverse effects associated with this ma-
neuver include pulmonary hypertension
13
in the fetus. A comorbid condition associ-
ated with NSAIA administration in this setting is nephrotoxicity of the fetus,
15
although a single injection does not seem to cause adverse effects. Earlier reports
on adverse effects of NSAIAs in human medicine are inconsistent; however, there
does seem to be some association of these drugs with teratogenesis,
16
especially dur-
ing the first trimester, during which much fetal organogenesis takes place.
17
Orofacial
clefts in the fetus may also be associated with NSAIA administration.
18
From earlier
reports, a renal embryopathy syndrome in babies when mothers were administered
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Table 1
Analgesic dosages for pregnant cats and dogs
Opioid Agonists Dose (mg/kg) Route of Administration
Dosing Interval
(Hours)
Morphine Dog:
0.1–0.5 Extremely slow IV bolus 1–4
0.1–0.5 IV per hour CRI
0.5–1 IM, SC 1–4 (6)
0.1–0.3 Epidural 4–8 (12)
0.5–2 PO titrate to effect 4–6
Cat: IV, extremely slowly
0.05–0.2 IM, SC 2–6
Oxymorphone Dog:
0.02–0.2 IV 2–4
0.05–0.2 IM, SC 2–4 (6)
0.05–0.3 Epidural 4–6
Cat:
0.02–0.1 IV 2–4
0.05–0.1 IM, SC 2–4
Hydromorphone Dog:
0.04–0.2 IV 4–6
0.05–0.2 (extreme cases) IM, SC 4–6
Cat:
0.02–0.1 (extreme cases) IV 4–6
0.05–0.1 IM, SC 4–6
Oxycodone Dog:
0.1–0.3 PO 6–8
Methadone Dog and cat:
0.1–0.5 IV, IM, SC 2–4
Codeine Dog:
1–2 PO titrate to effect 6–8
Cat:
0.5–1 PO titrate to effect 12
Naloxone is an opioid antagonist, and it should always be available when opioids are used. The
dose depends on the administered opioid, dose, and duration of action. Because the dose required
for reversal is never known, the author starts by slowly titrating naloxone intravenously in 0.004- to
0.04-mg/kg (0.01–0.1 mL of 0.4-mg/mL solution) increments until the desired clinical response is
achieved. For easy titration, one can combine naloxone, 0.1 to 0.25 mL, with 0.9% saline
(10 mL). It may be necessary to redose at varying intervals, because the duration of opioid action
is longer than that of naloxone.
The dosages given in Table 1 are those recommended for the nonpregnant cat or dog and are
given here as a guide. The dose used in the pregnant animal should be dosed according to lean
(nonpregnant) weight; however, the goal is to relieve pain, so titration to effect would be the
most prudent method of dosing.
Abbreviations: IM, intramuscular; IV, intravenous; PO, per os; SC, subcutaneous.
Adapted from Mathews KA, editor. PAIN HURTS: how to understand, recognize, treat, and stop
[CD-ROM ]. Guelph, Ontario, Canada: Jonkar Computer Systems; 2003; with permission.
Analgesia for Pregnant and Pediatric Dogs and Cats 129 5
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indomethacin for more than 48 hours has been recognized.
19
The NSAIAs may also
have adverse effects on the reproductive tract and fetus because they block prosta-
glandin activity, resulting in cessation of labor and disruption of fetal circulation.
20
Other complications include transient renal insufficiency that can measurably
decrease fetal urinary output and lead to oligohydramnios; fetal abnormalities in
hemostasis that extend into the neonatal period, predisposing to intraventricular hem-
orrhage; and reduction in mesenteric blood flow predisposing to necrotizing enteroco-
litis.
21
More recent studies have identified the importance of COX-2 for maturation of
the embryologic kidney; potential placental transfer of the NSAIAs may cause arrest of
nephrogenesis in the fetus.
22
NSAIDs should not be administered to pregnant animals.
A single injection after cesarean section is acceptable, however, and is frequently ad-
ministered at the author’s institution.
Because COX-2 induction is necessary for ovulation and subsequent implantation
of the embryo,
20
NSAIAs should be avoided in breeding female animals during this
stage of the reproductive cycle. A recent study in women taking aspirin, other NSAIAs,
or acetaminophen showed an 80% miscarriage rate in those exposed to aspirin or
other NSAIAs for 7 days or longer but not in those exposed to acetaminophen during
the first 20 weeks of gestation. Because acetaminophen has a different mechanism of
action than other NSAIAs, this may be why miscarriage did not occur.
23
The 20 weeks
of gestation in human beings may equate to approximately 4 to 5 weeks in dogs and
cats.
Because there are no studies specifically examining the safety or potential adverse
effects of the more recently approved NSAIAs in veterinary medicine (eg, meloxicam,
carprofen, deracoxib, firocoxib, tolfenamic acid, ketoprofen) to pregnant cats or dogs,
it is suggested that administration be restricted to a single dose after cesarean sec-
tion. Based on the Ontario Veterinary College (OVC) experience over several years,
where meloxicam, 0.1 mg/kg, administered intravenously is administered after cesar-
ean section, follow-up by the theriogenology department has not noted abnormalities
potentially associated with NSAIAs in the offspring at any time, suggesting that other
similar veterinary-approved NSAIAs would also be appropriate. The COX-2–specific
NSAIAs have not been used in this setting at the OVC, however.
Ketamine
Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, rapidly crosses the
placenta to the fetus in animals and human beings.
1
Ketamine has become a useful
adjunctive analgesic for severe pain in hospitalized patients. In this setting, it is nec-
essary to administer ketamine as a constant rate infusion (CRI) because of its short
duration of action. There are no reports in the veterinary or human literature examining
the effects on the mother or fetus at doses used in this setting (0.2–1.0 mg/kg/h).
Based on ketamine’s effect on uterine contractions and tone, maternal discomfort
and potential for miscarriage may be a concern. When administered at anesthetic dos-
ages, no teratogenic or other adverse fetal effects have been observed in reproduction
studies during organogenesis and near delivery with rats, mice, rabbits, and dogs.
1
Doses of 2 mg/kg administered to mothers before delivery resulted in profound respi-
ratory depression and increased muscle tone of the infant at birth, however; lower
doses (0.25–1.0 mg/kg) were not associated with these complications. In human be-
ings, low doses (0.275–1.1 mg/kg administered intravenously) of ketamine increased
uterine contractions, whereas higher doses (2.2 mg/kg administered intravenously) re-
sulted in a marked increase in uterine tone.
1
The effect of ketamine on intrauterine
pressure varies depending on the stage of pregnancy. In full-term human patients,
a 2-mg/kg intravenous dose of ketamine did not increase intrauterine pressure;
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however, the same dose given to women before termination of an 8-to 19-week preg-
nancy increased intrauterine pressure, intensity, and frequency of contractions. The
higher doses resulted in increased maternal systolic and diastolic pressure, and the
low doses had no effect on fetal blood pressure.
1
Neurobehavioral tests demonstrate
that human infants are depressed for up to 2 days after ketamine administration. If the
anesthesia induction-to-delivery interval is less than 10 minutes, fetal abnormalities
are greatly reduced.
1
ANALGESIA FOR NURSING MOTHERS
Occasionally, nursing mothers require a surgical procedure or sustain injuries that are
painful and require analgesic therapy. In addition to the humane aspect of treating the
mother, analgesia is also important because a litter of pups or kittens may aggravate
the painful state and may trigger aggression in the mother toward the pups or kittens.
Clearly, analgesics must be administered; however, there is a lack of information on
analgesic administration to lactating dogs or cats in the clinical setting. In addition
to the pharmacokinetics of transfer and concentration of the various analgesics in
breast milk, consideration must be given to the effects that the various analgesics
may have on different stages of maturity of the puppies and kittens (ie, the neonate
would potentially be more susceptible as a result of the immaturity of metabolizing
functions). Characteristics of a drug that would facilitate secretion into milk are high
lipid solubility, low molecular weight, and the non-ionized (charged) state. It is esti-
mated that the neonate receives approximately 1% to 2% of the maternal dose of
a drug.
24
The two classes of analgesics commonly used in veterinary patients are opi-
oids and NSAIAs. These drugs are excreted in the milk; however, in most instances in
people, the quantity is small. Nevertheless, there are differences that are important to
note. Unfortunately, this information is not available for the commonly prescribed vet-
erinary analgesics. Citations herein are therefore restricted to human and laboratory
animal studies.
Nonsteroidal Anti-Inflammatory Analgesics
A potential concern regarding the administration of NSAIAs immediately after cesar-
ean section, or even natural birth, is hemorrhage when the COX-1–preferential or
COX-1–selective NSAIAs are used (ie, aspirin, ketoprofen, ketorolac, naproxen, ibu-
profen). The continual presence of COX-2–preferential or COX-2–selective NSAIAs
in milk may inhibit maturation of the kidney in puppies or kittens, because COX-2 is
important in nephron maturation.
22
Complete maturation of the embryologic kidney
does not occur until approximately 3 weeks after birth,
25
and normal function does
not occur until approximately 6 to 8 weeks of age.
26–28
Because most NSAIAs may
not be lipid soluble, are highly protein bound to plasma proteins, and may be present,
to a great degree, in an ionized form in the plasma, theoretically, only a small amount
may appear in breast milk, and thus would be safe to use. The NSAIAs have different
characteristics that determine their secretion into milk and the metabolism and excre-
tion in the suckling animal, however. For example, the low lipid solubility of the NSAIAs
examined (ie, aspirin, ibuprofen, naproxen) results in a small amount being secreted in
the milk; however, celecoxib has a high lipid solubility, which predicts potential ease of
passage across biologic membranes.
29
It is suggested that celecoxib would readily
pass through the mammary epithelium.
30
The molecular weight indicates that it is
too large to pass through the pores of the mammary epithelium but could pass through
the membranes.
30
Two breast milk peaks for the presence of celecoxib occurred at
5 and 35 hours after the last oral dose given to a human patient. The reason for the
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second peak is not clear and requires further investigation. Other NSAIAs, such as
those with a long half-life (ie, naproxen, sulindac, piroxicam), can accumulate in the
infant with prolonged use.
31
Meloxicam was excreted in the milk of rats at concentra-
tions higher than those in plasma.
32
Acetaminophen is safe for use in human mothers,
but aspirin should only be used occasionally, and for brief periods, because human
infants eliminate salicylates slowly.
24
This would likely apply to kittens also. It is well
established that acetaminophen cannot be administered to cats. It has been sug-
gested that the single use of an NSAIA is safe in nursing human mothers.
33
Based
on these individual characteristics of the NSAIAs prescribed for human mothers, stud-
ies investigating passage into breast milk of veterinary-approved drugs in cats and
dogs are necessary to recommend guidelines for therapy. Until such studies are per-
formed revealing no adverse affect on renal maturation and function at maturity, it is
recommended that NSAIAs be reserved for single-dose use only after cesarean sec-
tion. This recommendation is based on the experience of the theriogenologist at the
OVC, where a single injection of meloxicam (COX-2–preferential NSAIA) is adminis-
tered after cesarean section.
Opioids
As with the NSAIAs, there are no studies investigating the administration of various
opioids in lactating cats and dogs. In the laboratory setting, morphine administration
to mice with newborn pups may result in altered maternal behavior;
34
however, these
changes have not been reported in the cat or dog despite the frequent administration
of opioids around the time of surgery.
7
The author has not noted abnormalities with the
mother or puppies after opioid administration for perioperative orthopedic or soft tis-
sue pain; the mothers are attentive and the puppies are playful. The lipid solubility of
the opioid influences its appearance in the milk; therefore, a more hydrophilic opioid,
such as morphine, may appear in smaller amounts than a more lipid-soluble opioid,
such as meperidine. A single dose of pethidine (meperidine) or morphine administered
to nursing mothers did not seem to cause any risk to the suckling infant; however, re-
peated administration of pethidine, in contrast to morphine, had a negative impact on
the infant.
33
Similarly, in another study, after receiving meperidine or morphine for
3 days after cesarean section, the babies of mothers receiving meperidine were
less responsive than the babies of mothers receiving morphine.
35
This may be related
to delayed metabolism or metabolites of the opioid rather than to a high concentration
through the milk, however. Short-term use of codeine in nursing mothers was also
noted to be safe; however, infant plasma samples 1 to 4 hours after feeding (20–24
minutes after administration to the mother) showed codeine levels to be higher than
those of morphine.
36
Intrathecal administration of morphine to a woman, before and
for 7 weeks after the birth of her baby, proved to be safe with no alterations in sleep,
arousal behavior, or general development in the infant.
37
Prolactin
38
and oxytocin
39
levels may be altered after morphine administration; however, there was no apparent
clinical effect in milk production or infant feeding in a human trial. A 15-day study of
human mothers on a methadone maintenance program, receiving 40 to 105 mg/d,
concluded that methadone is ‘‘safe’’ to use in breastfeeding mothers.
40
This was
based on detection of a total relative dose of less than 5% for both the S-methadone
and R-methadone enantiomers in the neonates. An arbitrary cutoff level of predicted
infant exposure of less than 10% of the maternal dosage is recommended. Metha-
done should be administered only parentally in dogs and cats. Butorphanol in human
beings passes into breast milk in concentrations paralleling levels in maternal serum.
41
At doses of 2 mg per person (estimated at 60–70 kg) every 6 hours, the American
Academy of Pediatrics considers butorphanol compatible with breastfeeding;
42
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however, as with any analgesic, allowing suckling to occur after peak levels of drug
have waned is advised. Hydromorphone hydrogen chloride (HCl) at a dose of 2 mg ad-
ministered intranasally to breastfeeding human mothers revealed a rapid distribution
from plasma into breast milk; however, the drug did not partition into the milk fat. It
was estimated that the infant would receive approximately 0.67% of the maternal
dose, which is considered a limited exposure.
43
Rather than withhold opioid analgesic therapy because of potential concern for the
puppies and kittens, when there is no published evidence to support this, administra-
tion of analgesics (Table 2) with observation of behavior is suggested. To prevent po-
tential for drug side effects, avoid nursing during peak drug levels; when possible, time
nursing immediately before the next dose, and avoid sedatives with long half-lives.
44
Should lack of vigor or respiratory depression attributable to opioid administration
occur in puppies or kittens, one drop of naloxone (0.4 mg/mL) under the tongue,
with titration to effect, should reverse these adverse effects.
Ketamine
No reports on the passage of ketamine into breast milk were found.
ANALGESIA FOR PEDIATRIC PATIENTS
An important fact to consider is that an unmanaged painful experience, especially
when the nervous system is developing, may have a permanent negative impact on
the animal. Studies in neonates and infants have revealed that when anesthesia or
analgesia was withheld during circumcision, altered pain sensitivity and increased
anxiety occurred with subsequent painful experiences, such as vaccination, when
compared with children who had undergone circumcision but received local anesthe-
sia.
45
Such studies suggest that infants retain a ‘‘memory’’ of a previous painful expe-
rience and their response to a subsequent painful stimulus is altered. This has also
been shown in laboratory animals,
46
and there is no reason to believe this to be any
different in cats and dogs.
In this discussion, the term pediatric generally refers to the first 6 months of life.
Because of important physiologic changes that occur during this time frame, a further
demarcation is defined for this review: neonatal (0–2 weeks), infant (2–6 weeks), wean-
ling, (6–12 weeks), and juvenile (3–6 months). This distinction is made to make the
reader aware of the metabolic changes that are occurring during these periods of
maturation.
47
Animals between 3 and 6 months of age seem to require adult dosing
regimens to effect analgesia.
There tends to be apprehension in administering analgesic drugs, especially opi-
oids, to young animals because of the often cited ‘‘decreased drug metabolism and
high risk for overdose.’’ Although this may be a potential concern in the neonate, it
is not necessarily so through all stages of maturation. Based on the human literature,
the analgesic requirement may be higher at a certain stage of development, especially
in the pediatric human patient, than in the adult.
48
Children 2 to 6 years of age have
greater weight-normalized clearance than adults for many drugs. Higher rates of
drug metabolism by cytochrome P-450 in children compared with adults have been
attributed to a larger liver mass per kilogram of body weight rather than to age-related
changes in intrinsic enzyme catalytic rates.
49
More rapid clearance in children may re-
quire more frequent dosing.
50
Although there are no reports in the veterinary literature
suggesting that increased dosing should be considered in the young cat or dog, per-
sonal experience with intensive monitoring of the rare young (4–6-month-old) animal
that inadvertently received 10 times the recommended opioid dose revealed no
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adverse effects; on the contrary, these animals seemed to be quite comfortable. This
is not to suggest that the opioid dose should be increased but to emphasize that
administering the analgesic to effect, rather than at a predetermined dose, is the
most important method by which to manage pain (Table 3). Opioids can be reversed
with naloxone (see Table 3) should there be clinical evidence of central nervous sys-
tem (CNS) depression with associated respiratory depression, hypotension, and
Table 2
Analgesic dosages for nursing mothers
Opioid Agonists Dose (mg/kg) Route of Administration
Interval
(Hours)
Morphine Dog:
0.1–0.5 Extremely slow IV 1–4
0.1–0.5 IV per hour CRI
0.5–1 IM, SC 1–4 (6)
0.1–0.3 Epidural 4–8 (12)
0.5–2 PO titrate to effect 4–6
Cat:
0.05–0.2 IV, extremely slowly 2–6
0.5–1 IM, SC 8–12
Hydromorphone Increased
temperature frequently
noted with administration
in cats
Dog:
0.04–0.2 IV 4–6
0.05–0.2 IM, SC 4–6
Cat:
0.04–0.1 IV 4–6
0.05–0.1 IM, SC 4–6
Oxycodone Dog:
0.1–0.3 PO 6–8
Methadone Dog and cat:
0.1–0.5 IV, IM, SC 2–4
Fentanyl Cat and dog:
0.001–0.0051IV loading 0.5–1
0.001–0.005 IV for 20–60 minutes CRI
0.05 anesthesia IV for 60 minutes CRI
Fentanyl transdermal
patch
Should be avoided due
to potential ingestion
by puppies or kittens
Codeine Dog:
1–2 PO titrate to effect 6–8
Cat:
0.5–1 PO titrate to effect 12
The dosages given here are those recommended for the nonlactating cat or dog and are given here
as a guide. The dose used in these animals should be that required to relieve pain; thus, titration to
effect would be the most prudent method of dosing.
For opioid reversal, see naloxone in Table 1.
Abbreviations: IM, intramuscular; IV, intravenous; PO, per os; SC, subcutaneous.
Adapted from Mathews KA, editor. PAIN HURTS: how to understand, recognize, treat, and stop
[CD-ROM ]. Guelph, Ontario, Canada: Jonkar Computer Systems; 2003; with permission.
Mathews
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bradycardia. If bradycardia (heart rate <60 beats per minute) is noted and is associ-
ated with poor perfusion only, glycopyrrolate may be administered rather than revers-
ing the opioid. Increasing the heart rate in this setting increases cardiac output.
Maintaining a normal heart rate to ensure appropriate cardiac output is important in
pediatric patients. Most animals that are comfortable and sleeping have low heart
rates, especially the larger breeds. This does not warrant concern. Because the effects
of the opioid occur quite rapidly after administration, it is wise to monitor for potential
adverse effects rather than to reverse a ‘‘potential’’ problem that may not happen.
Neonates, and potentially infants, need to be considered separately from the wean-
ling or juvenile patient when considering analgesics.
7
Neonates do feel pain, and the
nociceptive threshold may be lower than in the adult. This has been attributed to a po-
tential delay in development of the descending inhibitory mechanism. Because of the
slower development of some neurotransmitters or receptors, certain drugs may not be
effective at this stage of development. Because the NMDA system seems to be under-
developed in the neonate, ketamine may not be effective. In a review of sedatives and
analgesics used in children between the ages of 1–18 years of age, ketamine com-
bined with a benzoiazepine was noted to be a safe and effective analgesic/sedative
combination for emergency procedures.
51
Neonates have reduced clearance of
many drugs as compared with infants, children, and adults largely because of the
(1) greater water composition of their body weight, (2) larger fraction of body mass
that consists of highly perfused tissues, (3) lower plasma concentration of proteins
that bind drugs, and (4) incomplete maturation of their hepatic enzyme systems.
52
The hepatorenal system continues to develop until 3 to 6 weeks of age; this may result
in reduced metabolism and excretion, which may require alterations in dosing and
dosing intervals.
47
For all young animals, the presence of milk in the stomach may in-
hibit the absorption of some drugs, potentially resulting in lower blood levels.
Opioids
Lower doses of fentanyl or morphine are required for analgesia in the neonate when
compared with the 5-week-old puppy.
52
Extremely young puppies are also more sen-
sitive to the sedative and respiratory depressant affects of morphine, and it is recom-
mended that fentanyl may be a more suitable opioid in the extremely young, especially
the neonate.
53,54
This would likely apply to kittens. Sedative and opioid combinations
should be avoided in this young age group, because the sedation is extremely pro-
found. If further sedation is required, a low dose of the sedative may be administered
after the opioid has had time for full effect. Fentanyl was found to be an effective an-
algesic in children between 1-18 years of age undergoing emergency room proce-
dures. The addition of midazolam to fentanyl increased sedation and reduced
respiratory rate.
51
Morphine is the standard opioid for relief of severe pain in children; however, meper-
idine, fentanyl, and sufentanil are also administered when appropriate.
52,55,56
In veter-
inary patients, others have recommended administering half of the usual adult dose of
these agents to puppies and kittens when used as a premedication before anesthe-
sia.
57
For use as an analgesic, however, this may not be appropriate. Based on human
pediatric studies, dosing depends on the degree of pain and the phase of matura-
tion.
48
Neonates require less; however, animals a few weeks old may require an adult
dose regimen. Starting at lower dosages and increasing to effect is recommended.
Reversal of any adverse effects may be titrated using naloxone (see Table 3). The fol-
lowing dosing recommendations are ranges published for dogs and cats. Fentanyl
transdermal patches and fentanyl ‘‘lollipops’’ (transmucosal) are administered to
Analgesia for Pregnant and Pediatric Dogs and Cats 1301
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Tabl e 3
Analgesic dosages for pediatric patients
Drug Species
Dose (mg/ kg) (Lower Dosages in
Patients less than 4 Weeks of Age)
Route of Administration (SC Suggested
for Patients less than 4 Weeks of Age)
Interval
(Hours)
Mild to moderate pain
Opioid agonists
Morphine Dog
0.1–0.5 IM, SC, extremely slowly IV 1–4
0.051IV, SC per hour CRI
0.251PO titrate to effect 4–6 (8)
Cat
0.05–0.1 IM, SC 1–4
0.0251IV, SC per hour CRI
0.251PO titrate to effect 4–6 (8)
Methadone Dog and cat 0.1–0.5 IV, IM, SC 1–4
Fentanyl Dog and cat 0.002–0.010 IV loading 0.5–1
0.001–0.005 IV/20–60 min CRI
Meperidine Dog and cat 2–5 IM 0.5–1
Opioid agonist-antagonists
Butorphanol Dog 0.1–0.2 IV, IM, SC 1–4
Cat 0.1–0.2 (or to effect) IV, IM, SC 1–4
Dog and cat 0.05–0.011IV, SC per hour CRI
Opioid partial agonists
Buprenorphine 0.005–0.010 SC w6
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Moderate to severe pain
Opioid agonists
Morphine Dog 0.5–11IM, SC, extremely Slowly IV 1–4
0.051IV, SC per hour CRI
0.51PO titrate to effect 4–6 (8)
Cat 0.1–1 IM, SC 1–4
0.051IV, SC per hour CRI
0.51PO titrate to effect 4–6 (8)
Methadone Dog 0.5-11IM, SC, IV 1–4
Hydromorphone Dog and cat 0.05–0.1 IV, IM, SC 2–4 (6)
Fentanyl Dog and cat 0.005–0.0101IV loading 0.5–1
0.001–0.005 IV for 20–60 minutes CRI
Meperidine Dog and cat 2–5 IM 0.5–1
Sedatives
Midazolam Cat and dog 0.05–0.1 IV, IM Dog up to 6 hours
Cat can be >6 hours
Diazapam Cat and dog 0.05–0.1 IV Dog up to 6 hours
Cat can be >6 hours
Acepromazine Cat and dog 0.01–0.025 IM, SC 2–6
Naloxone is an opioid antagonist. Naloxone should always be available when opioids are used. The dose required depends on the administered opioid, dose,
and duration of action. Because a definitive dose for reversal is never known, the author starts by slowly titrating naloxone intravenously in 0.004- to 0.04-mg/kg
(0.01–0.1 mL of 0.4-mg/mL solution) increments until the desired clinical response is achieved. For easy titration, combine naloxone, 0.05 to 0.1 mL, with 0.9% saline
(10 mL). You may have to redose at varying intervals, because the duration of opioid action is longer than that of naloxone.
Abbreviations: IM, intramuscular; IV, intravenous; PO, per os; SC, subcutaneous.
Adapted from Mathews KA, editor. PAIN how to understand, recognize, treat, and stop [CD-ROM ]. Guelph, Ontario, Canada: Jonkar Computer Systems; 2003;
with permission.
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children.
56
No veterinary studies are available in this young group of patients assess-
ing these routes of administration.
Nonsteroidal Anti-Inflammatory Analgesics
The NSAIAs are not recommended for animals less than 6 weeks of age based on
developing hepatorenal systems. The COX-2 is required for renal maturation and
sodium and water balance at the level of the kidney. It is important to ensure
that this system is fully developed before administration of NSAIAs. Therefore, until
studies confirm the safety of these agents in animals less than 6 yo 8 weeks of age,
their use should be avoided.
Sedatives
Sedatives should be used with caution in young animals, especially when they are less
than 12 weeks of age.
58
The phenothiazine tranquilizers (ie, acetylpromazine) undergo
little hepatic biotransformation and may cause prolonged CNS depression. These
agents are not analgesic; in fact, they may mask an increase in the level of pain if
analgesics are not coadministered. These drugs induce peripheral vasodilation, and
hypotension and hypothermia may result; however, this is dose dependent. If these
drugs are required and there may be occasion for their use, the dosage for acetylpro-
mazine should be reduced to 0.005 to 0.025 mg/kg administered intramuscularly or
subcutaneously. It is advised that the concentration of 10-mg/mL commercial product
be diluted to 1 mg/mL before withdrawing for administration to facilitate accurate dos-
ing. Opioids have sedating effects, especially in young animals; therefore, if these
drugs are required, the addition of a sedative may not be necessary in animals younger
than 4 months.
LOCAL ANESTHETICS FOR ALL AGE GROUPS: GENERAL CONSIDERATIONS
The most frequently used local anesthetic in human medicine is lidocaine, and this
also seems to be the case in veterinary medicine. Infiltration of lidocaine is extremely
painful even with 27- to 30-gauge needles, especially in the neonate or pediatric
patient.
55
To reduce pain, buffering, warming (37–42C), and slow administration
are recommended. Buffering can be accomplished by mixing 1% lidocaine with so-
dium bicarbonate at a 10:1 ratio (1% lidocaine at a rate of 1 mL combined with sodium
bicarbonate at a rate of 0.1 mEq [0.1 mL of 1 mEq/mL]). Because most veterinary prac-
tices have a 2% solution, this can be diluted to a 1:1 ratio with 0.9% sodium chloride
(1% 510 mg/mL) and mixed with sodium bicarbonate to a further 10:1 ratio (lidocaine/
sodium bicarbonate) before administration. It might be advisable to use a maximum
dose of lidocaine in kittens of 3 mg/kg in the neonate to 6 mg/kg in the older pediatric
patient and a dose of 6 mg/kg in the neonatal pup to 10 mg/kg in the older pediatric
patient. The lower dose is required because of the immaturity of peripheral nerves and
not because the younger animals are at any greater risk for toxic side effects. This
dose should be diluted in 0.9% saline for accurate dosing, ease of administration,
and distribution over the site. Bupivacaine may also be used with a 2-mg/kg maximum
dose in the older kitten and puppy, with half of this dose advised for the neonate and
weanling. Buffering of a 0.5% bupivicaine solution (5 mg/mL) requires a 20:1 mixture
with sodium bicarbonate, 1 mEq/mL (w0.5 mL bupivacaine and 0.025 mL sodium
bicarbonate), and warming as described for lidocaine.
Previously, the most frequently used topical cream local anesthetic in our institution
was EMLA (eutectic mixture of local anesthetic) cream. This is a prescription-only mix-
ture of lidocaine, 2.5%, and prilocaine, 2.5%, combined with thickening agents to form
Mathews
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an emulsion (EMLA cream; AstraZeneca LP, Wilmington, Delaware). Dosing is calcu-
lated in a similar manner to the injectable formulation. Local anesthetic concentrations
are described as weight by volume (grams per 100 mL) or weight by weight (grams per
100 g); therefore, the 2.5% cream is 2500 mg per 100 g (25 mg/g). This product is not
sterile and should be used only in intact skin to provide anesthesia for intravenous
catheter placement, blood collection, lumbar puncture, and other minor superficial
procedures. EMLA cream should be covered with an occlusive dressing for at least
30 minutes and preferably longer. In children, the peak effect is at 2 hours; however,
the author’s experience in animals is that 30 minutes facilitates such procedures as
jugular catheter placement using the Seldinger technique; however a longer dwell
time might be necessary in the more active animal. In one veterinary study, there
was no systemic uptake of the components of EMLA cream and its use seemed to
be effective in preventing signs of discomfort during jugular catheter placement.
59
Another product is an over-the-counter liposome-encapsulated formulation of 4%
lidocaine (ELA-Max or L.M.X; Ferndale Laboratories, Ferndale, Michigan).Transdermal
absorption did occur after application of 15 mg/kg of this product, but plasma concen-
trations remained significantly lower than toxic values.
60
The area was covered to pre-
vent licking, with subsequent absorption through the oral mucous membrane and
concerns for lidocaine toxicity. A similar product containing 4% lidocaine (MAXILINE
4; Ferndale Laboratories, Ferndale, Michigan) is currently used at the author’s institu-
tion and is preferred to EMLA cream, because the local anesthetic effect occurs faster.
This product is only used in intact skin. Another solution, in gel form, is a mixture of 4%
lidocaine, 0.1% epinephrine, and 0.5% tetracaine, which can be applied to broken or
intact skin.
55
Because of the epinephrine content, this should be avoided in end-arte-
rial regions and mucous membranes. The author has no experience with the use of this
product, and there are no published pediatric veterinary studies. The advantage with
these products is that no injection is required, but they should be covered after appli-
cation for approximately 30 minutes.
Two percent lidocaine is also available in a sterile gel in a cartridge and is useful for
sterile local desensitization. This is frequently used at the author’s institution for desen-
sitization of the vaginal vault before urinary catheter placement in female cats and dogs,
and it may also be applied to the penis before urinary catheter placement.
CONCLUSIONS
The opioids seem to be the safest class of analgesic in this population of veterinary
patients. Specific drug selection, dosage, and timing of administration all have to be
considered. Although the NSAIAs may seem safe in certain settings in the human pa-
tient, their use in veterinary patients should be withheld until studies specifically inves-
tigating the veterinary-approved drugs prove their safety.
SUMMARY
Little information on the approach to analgesia in pregnant, nursing, or extremely young
animals is available in the veterinary literature. The unique physiologic characteristics in
this group of patients must be considered when selecting analgesics. As with mature
cats and dogs, the origin and severity of pain in this group of animals may be similar;
however differences do exist. The diagnosis and assessment of pain in pregnant and
nursing animals is based on the problem at hand and is similar that of mature animals.
The diagnosis in the extremely young, however, may be more challenging but should be
suspected based on history and clinical signs. Response to analgesic therapy is ad-
vised in all animals to confirm the presence and degree of pain. Various analgesics
Analgesia for Pregnant and Pediatric Dogs and Cats 1305
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and analgesic modalities are discussed, with emphasis placed on preference and cau-
tion for each group. Management of pain is extremely important in all animals but espe-
cially in the extremely young, in which a permanent hyperalgesic response to pain may
exist with inadequate therapy. Inappropriate analgesic selection in pregnant and nurs-
ing mothers may result in congenital abnormalities of the fetus or neonate. Inadequate
analgesia in nursing mothers may cause aggressive behavior toward the young. Review
of the human and veterinary literature on the various analgesics available for use in this
group of patients is discussed.
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